The present invention is drawn generally to a process for enhancing air quality and restoring the environment through the removal of elemental mercury from gases released to or present in the atmosphere, and drawn more specifically to a method for controlling mercury emissions from flue gases.
In recent years, research has been performed to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers and waste-to-energy plants. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg+2) form as, for example, mercuric chloride, (HgCl2), or in its elemental (Hg0) form as vaporous metallic mercury. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and various other factors.
The search for industrially acceptable methods for the capture of mercury from industrial flue gases has included a significant effort to determine how much mercury can be removed by existing, conventional air pollution control equipment, such as wet or dry scrubbers.
Accordingly, tests have been performed on several commercial scale and pilot scale wet scrubbers, which are designed for the capture of sulfur oxides and other acid gases. These tests have produced some expected and some surprising results. It was generally expected that the oxidized mercury would be easily captured and the elemental mercury would be difficult to capture. These expectations were based on the high solubility of mercuric chloride in water and the very low solubility of elemental mercury in water. This expectation was generally fulfilled.
The surprising result concerned elemental mercury. Repeated tests, during which the concentration of elemental mercury in the flue gas was measured, revealed that more elemental mercury was leaving the wet scrubber than was entering.
One postulate proposed to explain the cause of the elemental mercury generation in the wet scrubber is described for example, by the following general reactions:Mex+Hg+2→Mex+2+Hg02Mex+Hg+2→2Mex+1+Hg0
Me is any number of transition metals, such as Fe, Mn, Co, etc., or other metals, such as Sn, that may be present in one of several possible oxidation states, x. These or other chemically reducing species may result in elemental mercury generation.
Transition metal ions are generally present in wet scrubber slurries as impurities in the industrial applications of concern. Thus, as the mercuric chloride is absorbed, a portion reacts with and becomes reduced by trace levels of transition metals and metal ions and, because of its low solubility, the elemental mercury is stripped from the liquid and returned to the flue gas.
Most of the recent efforts to capture and remove mercury from the flue gas produced by coal-fired units have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
Alternatively, U.S. patent application Ser. Nos. 09/282,817 (“Use of Sulfide-Containing Gases and Liquors for Removing Mercury from Flue Gases”) and 09/464,806 (“Use of Sulfide-Containing Liquors for Removing Mercury from Flue Gases”), describe a means in a wet or dry scrubber to rapidly precipitate the oxidized mercury at the gas/liquid interface in the scrubber before it can be reduced by the transition metals. One of the most insoluble forms of mercury is mercuric sulfide (HgS), which in mineral form is cinnabar. Means for supplying a source of sulfide to react with the oxidized mercury include use of hydrogen sulfide (H2S) and/or aqueous sulfide ions. Thus, at the gas/liquid interface in the scrubber, the following reactions are proposed for the absorption and precipitation of ionized (oxidized) mercury (depending upon whether the sulfide is derived from hydrogen sulfide gas, aqueous sulfide ions, or some other sulfide ion source):S−2(aq)+HgCl2(g)→HgS(s)+2Cl−(aq)and/orH2S(g)+HgCl2(g)→HgS(s)+2H+(aq)+2Cl−(aq)
HgS has a solubility product of 3×10−52 and therefore precipitates essentially completely. The aqueous sulfide species is added to the scrubbing liquor of the scrubber and comes into contact with the mercury in the flue gas, such that HgS is formed when the mercury is absorbed into the liquor. Likewise, in the case of hydrogen sulfide gas, there is good reason to expect that the precipitation reaction proceeds faster than the reduction reactions. Specifically, in the case of the precipitation reaction, both reactants are well mixed in the gas phase. Thus, as they diffuse from the gas to the gas/liquid interface, both reactants can react instantly at the interface. By contrast, the reduction reactions require that the reactants, i.e., the Hg+2 and the transition metal ion or other chemically reducing species, diffuse in the liquid phase to a reaction plane in the liquid. Liquid phase diffusion is orders of magnitude slower than gas phase diffusion.
Therefore, using gas and/or aqueous sulfide species, the oxidized mercury will rapidly precipitate as cinnabar in the scrubber and thereby prevent the reduction of that mercury back to vaporous elemental mercury. The precipitation of mercury as cinnabar has a distinct advantage over other mercury sequestering methods in that it converts mercury to a very insoluble form. In this way, the mercury should be inert and effectively removed from the food chain.
However, the methods discussed above all have one significant limitation—the amount of the elemental mercury in the flue gas, Specifically, these methods all require the mercury to be in its oxidized state (such as HgCl2), but the relative amount of oxidized vs. elemental mercury species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and various other factors. Consequently, scrubbers treating a flue gas with only half of the mercury in an oxidized form and half in an elemental form will be limited to a total mercury removal of only about 50%. A method which permits complete removal of all mercury, both oxidized and elemental, would be welcome by the industry.
U.S. Pat. No. 5,009,871 describes a method in which chlorine is added to a scrubbing solution in a proper form to prevent the chemical reduction of absorbed mercuric chloride and mercury forming complex ions with chlorine. This method is specifically directed at the capture of gaseous mercuric chloride as found in waste incinerators. The method excludes the elemental mercury and does not address the fate of the mercury once it is in solution.
U.S. Pat. No. 4,443,417 describes a method and apparatus by which elemental mercury can be removed from a gas stream using chlorine as an oxidant. However, this process uses an acidic liquid containing sulfuric acid (H2SO4), hydrochloric acid (HCl) and hydrogen fluoride (HF) in a concentration of about 1% by weight, thereby requiring the handling of potentially dangerous materials. Furthermore, this method is not applicable to alkaline slurries of the type used for sulfur dioxide (SO2) removal from flue gases.
Zhao and Rochelle (“Mercury Absorption in Aqueous Hypochlorite,” published in August 1999) have shown that elemental mercury can be absorbed into aqueous hypochlorous solutions and that gas phase molecular chlorine (Cl2) assisted in the absorption of elemental mercury by an aqueous solution. However, this article simply demonstrates the feasibility of the reaction, and fails to mention any practical application for the reaction.
European Patent WO9958228 describes the addition of chlorine to flue gas for the purpose of oxidizing elemental (metallic) mercury vapor (as well as nitrogen oxides (NOx), SO2, and H2S) to form mercuric chloride, which is then absorbed by the sulfuric acid solution that results from the conversion of SO2 to H2SO4. The mercuric chloride is then precipitated using an alkali metal halogen salt such as potassium iodide (KI). According to this method, the chlorine must be injected into the flue gas at a temperature in excess of 100° C. where the mercury is oxidized in the gas phase and the mercuric chloride is absorbed into an acidic sulfuric acid solution. It specifically does not include alkali or alkaline slurries, the addition of chlorine to the aqueous phase, or the precipitation of the absorbed mercury as mercuric sulfide, while at the same time including the oxidation of H2S, which is viewed as a detrimental and unwanted reaction because sulfide species (H2S and/or aqueous sulfide species) are needed to assist in the sequestration of mercury.
In light of the foregoing, a method which permits selective or complete control of the removal of all mercury species from flue gases and/or which does not require high temperature injection schemes would be welcome by the industry. Likewise, a method which does not require the handling of dangerous materials and/or which selectively oxidizes elemental mercury is needed.